High temperature oxidation inhibitors for carbon-carbon friction materials

ABSTRACT

Coated articles ( 19 ) that comprise components, made of carbon fiber or carbon-carbon composites which may be configured, for example, as aircraft landing system brake discs. The components ( 10 ) are coated with a system that includes a phosphorus-containing undercoating ( 11 ) having a specified formulation and a boron-containing overcoating ( 12 ) having specified characteristics. The coated articles of the invention, e.g., aircraft brake discs, are protected against catalytic oxidation when the article is subjected to temperatures of 800° C. (1472° F.) or greater. Also, a method of protecting a component made of a carbon fiber or carbon-carbon composite simultaneously against catalytic oxidation (e.g., catalyzed by de-icer compositions) and high temperature non-catalytic oxidation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of copending application Ser. No.10/223,946, filed Aug. 20, 2002. Priority in accordance with 35 U.S.C.§120 is claimed to application Ser. No. 10/223,946. The entiredisclosure of application Ser. No. 10/223,946 is incorporated byreference into the present application.

FIELD OF THE INVENTION

This invention relates to a novel coating system for effecting a widespectrum of oxidation resistance in carbon-carbon composites and othergraphitic materials and to methods for the preparation of theoxidatively protected composites. This invention is particularlybeneficial in the field of aircraft braking systems.

BACKGROUND OF THE INVENTION

Carbon fiber or C—C composites which are useful for instance in airplanebraking systems are subject to oxidation and resultant weight loss (thatis, loss of mass). Oxidative weight loss of such carbon composites isgenerally retarded by coating articles made of the carbon compositeswith an antioxidant coating.

U.S. patent application Ser. No. 09/518,013 (Golecki), filed 3 Mar.2000, now U.S. Pat. No. 6,737,120 B1, relates to carbon fiber or C—Ccomposites that are useful in a variety of applications. Golecki teachesmethods of protecting such composites against oxidation by coating themwith fluidized-glass type mixtures. The fluidized-glass mixtures aremaintained as liquid precursors and are applied to components formed ofcarbon fiber or C—C composites. Once coated with the precursors, thecoated C—C components are heat-treated or annealed for one or morecycles through a series of gradual heating and cooling steps. Thiscreates glass coatings having thicknesses of about 1-10 mils. Thethicknesses of the glass coatings may be varied by varying thecomposition of the fluidized glass precursor mixtures, the number ofapplication cycles, and/or the annealing parameters.

The Golecki application teaches that the fluidized glass materials maycomprise such materials as borate glasses (boron oxides), phosphateglasses (phosphorus oxides), silicate glasses (silicon oxides), andplumbate glasses (lead oxides). These glasses may include phosphates ofmanganese, nickel, vanadium, aluminum, and zinc, and/or alkaline andalkaline earth metals such as lithium, sodium, potassium, rubidium,magnesium, and calcium and their oxides, and elemental boron and/orboron compounds such as BN, B₄C, B₂O₃, and H₃BO₃. By way of example,Golecki discloses a boron-containing liquid fluidized glass precursormixture that includes 29 weight-% phosphoric acid, 2 weight-% manganesephosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10weight-% boron, and 55 weight-% water.

The composites coated with boron-containing fluidized-glass typemixtures of Golecki—that is, coated articles comprising components madeof carbon fibers or carbon-carbon composites annealed at very hightemperatures, each component being covered by a glass coating ofapproximately 1-10 mil—are protected against uncatalyzed oxidation whenthe article is subjected to temperatures of 800° C. (1472° F.) orgreater, for instance at 1600° F. (871° C.). However, these compositesdo not show good resistance to some types of catalytic oxidation. Inparticular, for instance, such boron-containing glass-coated compositeshave been found to be subject to significant oxidative weight loss afterexposure to airport runway de-icer (potassium acetate).

U.S. patent application Ser. No. 09/507,414 (Walker and Booker), filed18 Feb. 2000, now U.S. Pat. No. 6,455,159 B1, likewise relates tocarbon-carbon composites and graphitic materials. The Walker and Bookerapplication has as objectives the protection of carbon/carbon compositesor graphites at elevated temperatures up to and exceeding 850° C. (1562°F.) and the reduction of catalytic oxidation at normal operatingtemperatures.

Walker and Booker achieve these objectives by employing a penetrant saltsolution which contains ions formed from 10-80 wt % H₂O, 20-70 wt %H₃PO₄, 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate, andup to 2 wt % B₂O₃. Their penetrant salt solutions also include at leastone of MnHPO₄.1.6H₂O, AlPO₄, and Zn₃(PO₄)₂, in weight-percentages up to25 wt %, 30 wt %, and 10 wt %, respectively.

While the Golecki coatings and the Walker and Booker coatings do providesignificant antioxidant protection for carbon composites, there remainsa need for antioxidant coating systems that provide a high level ofoxidation protection simultaneously in both high temperature andcatalyzed oxidation conditions.

SUMMARY OF THE INVENTION

The present invention provides coated articles that comprise components,made of carbon fiber or carbon-carbon composites which are annealed attemperatures in the range of 1600-2600° C. (2912-4712° F.). Thesearticles may be configured, for example, as aircraft landing systembrake discs.

In accordance with this invention, the component is covered by anundercoating comprising ions formed from 10-80 wt % H₂O, 20-70 wt %H₃PO₄, 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate, 0-2wt % B₂O₃, and 0-25 wt % MnHPO₄.1.6H₂O, 0-30 wt % AlPO₄, and 0-10 wt %Zn₃(PO₄)₂, provided that at least one of AlPO₄, MnHPO₄.1.6H₂O, andZn₃(PO₄)₂ is present. The undercoating preferably comprises ions formedfrom 20-50 wt % H₂O, 30-55 wt % H₃PO₄, 0-15 wt % MnHPO₄.1.6H₂O, 2-15 wt% AlPO₄, 0.5-2 wt % B₂O₃, 1-7 wt % Zn₃(PO₄)₂, and 10-20 wt % KH₂PO₄.This undercoating generally has a thickness of approximately 1-10 mil,and is annealed to the carbon composite at a temperature in the range of250-900° C. (482-1652° F.).

In further accordance with the present invention, thisphosphorus-containing undercoating is covered by a boron-containingglass overcoating of approximately 1-10 mil in thickness, which isannealed to the undercoating at a temperature in the range of 250-650°C. (482-1202° F.). The formulation used to make the boron-containingglass overcoating also preferably contains phosphoric acid. It isparticularly preferred that the boron-containing glass overcoatingcontains boron nitride and also contains boron carbide and/or elementalboron.

The resultant coated article, e.g., an aircraft brake disc, is protectedagainst catalytic oxidation, even when the article is subjected totemperatures of 800° C. (1472° F.) or higher.

The present invention also contemplates a method of protecting acomponent made of carbon fiber or carbon-carbon composite simultaneouslyagainst catalytic oxidation (e.g., catalyzed by CH₃COOK) and hightemperature non-catalytic oxidation, which method comprises the steps ofcovering said composite with an undercoating comprising ions formed from10-80 wt % H₂O, 20-70 wt % H₃PO₄, 0.1-25 wt % alkali metal mono-, di-,or tri-basic phosphate, 0-2 wt % B₂O₃, and 0-25 wt % MnHPO₄.1.6H₂O, 0-30wt % AlPO₄, and 0-10 wt % Zn₃(PO₄)₂, provided that at least one ofAlPO₄, MnHPO₄.1.6H₂O, and Zn₃(PO₄)₂ is present, said undercoating havinga thickness of approximately 1-10 mil, and subsequently covering theundercoating with a boron-containing glass overcoating having athickness of approximately 1-10 mil.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a carbon fiber or C—C substrate having acoating system formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is embodied by a carbon fiber or C—C compositecomponent that is coated with a two-layer anti-oxidant system. For abetter understanding of this invention, attention is directed to FIG. 1,wherein a coated carbon fiber or C—C article component is generallyillustrated at 19. A component 10 is covered by a protectiveundercoating 11 (phosphorus-containing glass) and the undercoating 11 iscovered by a protective overcoating 12 (boron-containing glass).

Component 10

Before the first fluidized glass formulation is applied to the C—Ccomposite component, the component may be fabricated into almost anydesired shape. The present invention is particular valuable with the C—Ccomposite component is an aircraft landing system brake disc.

Carbon-carbon composites are generally prepared from carbon preforms.Carbon preforms are made of carbon fibers, formed for instance ofpre-oxidized polyacrylonitrile (PAN) resins. These fibers can be layeredtogether to form shapes, such as friction brake discs, which shapes arethen heated and infiltrated with methane or another pyrolyzable carbonsource to form the C—C composite preforms. Carbon-carbon compositesuseful in accordance with the present invention typically have densitiesin the range of from about 1.6 g/cm³ through 1.9 g/cm³. Methods ofmanufacturing C—C composites are generally well known to those skilledin the art. A good reference in this area is: Buckley et al.,Carbon-Carbon Materials and Composites, Noyes Publications, 1993. Theentire contents of this publication are hereby expressly incorporated byreference.

For purposes of illustration only, the C—C composite component 10 may befabricated from woven fabric panes of pitch-based Amoco P30X carbonfiber tows in a harness satin weave or from a pitch-based Nippon XNC25in a plain weave. The tows are rigidized with a few weight-%carbon-containing resin, such as epoxy Novolac. The material is thencarbonized at a temperature in the range of 800-1000° C. (1472-1832° F.)and densified by carbon CVD. The resulting materials is then annealed inan inert gas at a temperature in the range of 1600-2600° C. (2912-4712°F.). This process creates a C—C composite component that is adaptablefor use in high temperature environments when it is properly protectedagainst oxidation. It is understood that the oxidation protectivecoating system of the present invention is applicable to C—C compositecomponents regardless of how the C—C composite components arefabricated.

Undercoating 11

The C—C component 10 is immersed or dipped in a liquid bath precursor offluidized glass for several minutes. Preferred precursors for use inapplying the undercoating layer in accordance with the present inventionare phosphoric acid-based penetrant salt solutions, which are describedin detail in U.S. patent application Ser. No. 09/507,414, filed 18 Feb.2000, the entire disclosure of which is hereby expressly incorporated byreference. A typical penetrant salt solution that can be used to formthe undercoating herein contains ions formed from 10-80 wt % H₂O, 20-70wt % H₃PO₄, 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate,and optionally up to 2 wt % B₂O₃. The typical penetrant salt solutionwill also include at least one of MnHPO₄.1.6H₂O, AlPO₄, and Zn₃(PO₄)₂,in weight-percentages up to 25 wt %, 30 wt %, and 10 wt %, respectively.

In accordance with this invention, the surface of the carbon-carboncomposite or graphitic material is treated with the penetrant solutionby painting, dipping, or other conventional application techniques.Subsequently the surface-treated material is cured at a temperature inthe range of 250-900° C. (482-1652° F.). Typically, the surface istreated with one to three coats of the penetrant solution, and thecuring step is generally accomplished in one to six hours.

The fluidized glass liquid precursor as described above is maintained ata temperature in the range of approximately 20-90° C. (68-194° F.).Component 10 may be rotated relative to the liquid precursor to improvethe wetting characteristics and uniformity of the coating. After theimmersion step is complete, the glass-coated component 10 is removed andannealed or heat-treated in a non-oxidizing environment. The annealingstep may be carried out with a relatively slow ramp-up and possible slowramp-down of the heating and cooling rates, respectively. For instance,the ramp-up rate may be on the order of 1-2 C° (1.8-3.6° F.) per minute.The temperature time cycle of the annealing process may vary. Forinstance, the heat treatment may include a gradual ramp up intemperature to about 250-350° C. (482-662° F.) at the aforesaid rate,which may then be followed by a soak wherein the temperature ismaintained in the range of 250-350° C. (482-662° F.) for approximately1-10 hours. Upon completion of this extended heating step thetemperature may be further increased, at the aforesaid rate, untilreaching a range of 550-650° C. (1022-1202° F.), at which point thetemperature is maintained in that range for approximately 1-10 hours.After completion of this second prolonged heat treatment, the componentmay be gradually cooled at a ramp-down rate on the order of 1-2 C°(1.8-3.6° F.) per minute until reaching ambient temperature.

These annealing procedures may make use of a flowing inert gas, such asnitrogen or argon. Alternatively, component 10 may be located in avacuum changer. In either case, upon completion of these annealingsteps, the fluidized phosphorus-containing glass coating is converted toa solid glass coating 11 completely enveloping C—C component 10.

Overcoating 12

The composite component 10 bearing the undercoating 11 is immersed orbathed in a fluidized boron-containing glass precursor. The overcoating12 may comprise ions formed from 20-60 wt % H₂O, 25-50 wt % H₃PO₄, 2-20wt % alkali metal hydroxide, 1-10 wt % alkali or alkaline earth metalmono-, di-, or tri-basic phosphate, 1-10 wt % boron nitride, and one orboth of 1-10 wt % elemental boron and/or 1-10 wt % boron carbide. Thesecomponents and their relative amounts are illustrative and not limiting.A more complete disclosure of boron-containing glass precursors usefulas the overcoating in accordance with the present invention may be foundin U.S. patent application Ser. No. 09/518,013, filed 3 Mar. 2000, theentire disclosure of which is hereby expressly incorporated byreference.

The following Table shows various typical embodiments of specificovercoating formulations that can be used in accordance with the presentinvention (with weights given in grams). B4 B5 B6 B7 Phosphoric 188.6188.6 188.6 188.6 acid Potassium 18 18 18 18 hydroxide Manganese 14.114.1 14.1 14.1 phosphate BN (boron 9.2 9.2 9.2 9.2 nitride) B (boron,67.3 101 33.6 0 elemental) B₄C (boron 0 0 43.0 86.0 carbide) Water 200200 200 200

Such overcoating formulations for use in this invention can be prepared,for instance, by: premixing the phosphoric acid with 128 grams of water,premixing the potassium hydroxide with 72 grams of water, combiningthese two aqueous mixtures, adding the manganese phosphate, and finallyadding the boron and/or boron compounds.

The C—C component 10 is immersed or dipped in a liquid bath precursor offluidized boron-containing glass for several minutes. The liquidprecursor is maintained at a temperature in the range of approximately20-90° C. (68-194° F.). The component 10 may be rotated relative to theliquid precursor to improve the wetting characteristics and uniformityof the coating. After the immersion step is complete, the glass-coatedcomponent 10 is removed and annealed or heat-treated in a non-oxidizingenvironment. The annealing step may be carried out with a relativelyslow ramp-up and possible slow ramp-down of the heating and coolingrates, respectively. For instance, the ramp-up rate may be on the orderof 1-2 C° (1.8-3.6F°) per minute. The temperature time cycle of theannealing process may vary. For instance, the heat treatment may includea gradual ramp up in temperature to about 250-350° C. (482-662° F.) atthe aforesaid rate, which may then be followed by a soak wherein thetemperature is maintained in the range of 250-350° C. (482-662° F.) forapproximately 1-10 hours. Upon completion of this extended heating stepthe temperature may be further increased, at the aforesaid rate, untilreaching a range of 550-650° C. (1022-1202° F.), at which point thetemperature is maintained in that range for approximately 1-10 hours.After completion of this second prolonged heat treatment, the componentmay be gradually cooled at a ramp-down rate on the order of 1-2 C°(1.8-3.6F°) per minute until reaching ambient temperature.

These annealing procedures may make use of a flowing inert gas, such asnitrogen or argon. Alternatively, component 10 may be located in avacuum changer. In either case, upon completion of these annealingsteps, the fluidized boron-containing glass coating is converted to asolid glass coating 12 completely enveloping and forming—with solidglass coating 11—a protective barrier against undesirable catalytic andnon-catalytic oxidation of C—C component 10. In other words, at thisstage, the composite component 10 is permanently enveloped within afluidized glass protective coating system (11, 12). The coating system(11, 12) comprises glass materials that are capable of at least someflowing with at least partial sealing of any pre-existing cracks thatmay be present in the C—C component.

Variability

The properties of the glass materials 11 and 12 may be tailored to thetemperature range over which and/or oxidation catalysts to which coatingsystem (11, 12) is designed to protect the composite component 10 fromdestructive oxidation. Likewise, the thicknesses of and numbers of glasscoatings applied to component 10 will depend on the method of applyingthe coating and the intended use for the coated article 19. If thecoated article will be subjected to sustained or repeated very hightemperatures, a number of separate sub-layers may be applied to make upundercoating 11 and/or overcoating 12. This antioxidant coating systemmay be used on a wide variety of carbon fiber or carbon-carbon compositearticles, including but not limited to aircraft landing system brakediscs.

Properties

The following Table elucidates unexpected properties shown by thepresently claimed coated articles. In the Table, (C—C)P-13K was acomposite with a phosphorus-containing coating, (C—C)B5 was a compositewith a boron-containing coating, (C—C)B5/P-13K was a composite having aboron-containing undercoating and a phosphorus-containing overcoating,and (C—C)P-13K/B5 was a composite having a phosphorus-containingundercoating and a boron-containing overcoating.

To determine weight loss on oxidation, coated C—C specimens were weighedand their dimensions were measured prior to being subjected to a flowingstream of dry air in an alumina tube while heated in a furnace having auniform hot zone sufficiently large to encompass the components. The dryflowing air was maintained at temperatures of 871° C. (1600° F.) to testfor uncatalyzed oxidation and 649° C. (1200° F.) to test for catalyzedoxidation. The results are reported in the Table. De-Icer CatalyzedOxidation² Uncatalyzed Oxidation¹ (potassium acetate) (C—C)P-13K 14.5%weight loss  0.6% weight loss (C—C)B5   1% weight gain   82% weight loss(C—C)B5/P-13K 49.5% weight loss (C—C)P-13K/B5  0.3% weight lossa. oxidation for 5 hours at 871° C. (160 0° F.)b. oxidation for 24 hours at 649° C. (12 00° F.)

As can be seen from the results present in the above Table, the P-13Kcoating provides good protection against catalyzed oxidative weight lossbut not against uncatalyzed oxidative weight loss and the B5 coatingprovides good protection against uncatalyzed oxidative weight loss butnot against catalyzed oxidative weight loss. Also, a composite having aboron-containing undercoating and a P-13K overcoating is not protectedagainst catalyzed oxidative weight loss. Surprisingly, however, acomposite having a P-13K undercoating and a boron-containing overcoatingis well protected against oxidative weight loss.

1. A coated article comprising a component made of carbon fiber orcarbon-carbon composite annealed at a temperature in the range of1600-2600° C. (2912-4712° F.), said component being covered by aphosphorus-containing undercoating comprising ions formed from 10-80 wt% H₂O, 20-70 wt % H₃PO₄, 0.1-25 wt % alkali metal mono-, di-, ortri-basic phosphate, 0-2 wt % B₂O₃, and 0-25 wt % MnHPO₄.1.6H₂O, 0-30 wt% AlPO₄, and 0-10 wt % Zn₃(PO₄)₂, provided that at least one of AlPO₄,MnHPO₄.1.6H₂O, and Zn₃(PO₄)₂ is present, said undercoating having athickness of approximately 1-10 mil, said undercoating being covered bya boron-containing glass overcoating of approximately 1-10 mil inthickness, wherein said coated article is protected against catalyticoxidation when the article is subjected to temperatures of 800° C.(1472° F.) or greater.
 2. The coated article of claim 1, configured asan aircraft landing system brake disc.
 3. The coated article of claim 1,wherein the phosphorus-containing undercoating comprises ions formedfrom 20-50 wt % H₂O, 30-55 wt % H₃PO₄, 0-15 wt % MnHPO₄.1.6H₂O, 2-15 wt% AlPO₄, 0.5-2 wt % B₂O₃, 1-7 wt % Zn₃(PO₄)₂, and 10-20 wt % KH₂PO₄. 4.The coated article of claim 1, wherein the boron-containing glassovercoating comprises boron nitride and also comprises boron carbideand/or elemental boron.
 5. The coated article of claim 4, wherein theformulation used to make the boron-containing glass overcoatingcomprises ions formed from 20-60 wt % H₂O, 25-50 wt % H₃PO₄, 2-20 wt %alkali metal hydroxide, 1-10 wt % alkali or alkaline earth metal mono-,di-, or tri-basic phosphate, 1-10 wt % boron nitride, and one or both of1-10 wt % elemental boron and/or 1-10 wt % boron carbide.
 6. The articleof claim 1, wherein the undercoating is annealed to the carbon compositeat a temperature in the range of 250-900° C. (482-1652° F.).
 7. Thearticle of claim 1, wherein the glass overcoating is annealed to theundercoating at a temperature in the range of 250-650° C. (482-1202°F.).